When I got my Trek Madone a month ago I marvelled at how it felt faster on the flats and I was able to sprint a whole lot better on it too. Given I am a sprint specialist in races, the Madone suits me. The power just goes down and I get maximum benefit for the watts I can produce. My Wilier just skips about and while it accelerates quickly, is generally skittish and doesn't put the power down as well.
I enjoy riding the Trek so much, I took it up a long climb on a Club ride last week. Felt good personally, hated the climb because of the bike. The Trek felt sluggish and heavy. Now, at 8.4kg, it isn't a weight-weenie lightweight albeit not particularly heavy either.
Yesterday I went on another Club ride and took my lighter, 5.8kg, Wilier. Having not ridden it since the arrival of the Trek, it was a revelation in acceleration on climbs and I set
PR's on every segment of the 900m high, 18km climb we did.
The lighter bike felt wonderful on the climb and really proved to me that each bike has its place. The heavier, more aero Trek is my sprint and flat route choice and the Wilier my climbing bike. It was always intended to be this way, of course, but only experiencing both properly could I know for certain that there is a tangible difference and not just something one wants to believe.
Edited to add: I'm not heavy either, 65kg. My ideal race weight would be a couple of kg's lighter but not much more than that at my age so I'm not carrying too much extra.